De Broglie derived his equation using well established theories through the following series of substitutions:
1. De Broglie first used Einstein’s famous equation relating matter and energy:
E=mc2 (0)
With
a) E = energy
b) M = mass,
c) C = speed of light
2. Using Planck's theory which states every quantum of a wave has a discrete amount of energy given by planck’s equation:
E=hv (1)
With
a) E= energy,
b) H= Plank’s constant (6.62607×10−34Js),
c) V= frequency
3. Since de Broglie believed particles and wave have the same traits, he hypothesized that the two energies would be equal.
mc2=hv (2)
4. Because real particles do not travel at the speed of light, De Broglie submitted velocity (v) for the speed of light (c).
mc2=hv (3)
5. Through the equation .λ De Broglie substituted vλ for v and arrived at the final expression that relates wavelength and particle with speed.
mv2=hvv (4)
Hence:
λ=hvmv2=hmv (5)
A majority of wave-particle duality problems are simple plug and chug via 5 with some variation of cancelling out units.